Prepreg is prepared by impregnating cloth, which is formed by interlacing warp and weft threads, with thermosetting resin such as epoxy resin, and subsequently drying it. A laminated board may be prepared by stacking one or more sheets of the prepreg, and then molding it by means of heating and pressing. Thereafter, a printed wiring board is prepared by forming a circuit on a face of the laminated board.
In this regard, the aforementioned cloth is prepared by interlacing parallel warp threads with parallel weft threads at a right angle to the weft threads, and the cloth is formed into a sheet elongated along a longitudinal direction of the warp threads. Moreover, the prepreg, which is prepared by impregnating the cloth as a substrate with thermosetting resin and drying it, is formed into an elongated sheet having a similar shape to the cloth. As shown in FIG. 6A, warp threads 110 are arranged parallel to a side edge “e” along a longitudinal direction of a prepreg sheet 180, and weft threads 120 are arranged at a right angle to the side edge “e” along the longitudinal direction of the prepreg sheet 180. Note that cloth 130 is embedded in the prepreg sheet 180, however, the warp threads 110 and the weft threads 120 of the cloth 130 are shown with solid lines in FIG. 6A.
A laminated board 190 is prepared by stacking at least one of the prepreg sheets 180, and subsequently molding a laminate by means of heating and pressing so as to harden resin 140 (e.g., thermosetting resin) penetrating into the cloth 130. Thereafter, a printed wiring board 200 is prepared by forming a circuit 150 with a predetermined pattern, on a face of the laminated board 190.
FIG. 6B shows the laminated board 190 and the printed wiring boards 200 prepared by forming the circuits 150 on the face of the laminated board 190. With forming the circuits 150 at multiple sites of the single laminated board 190 and thus cutting the laminated board 190 along peripheries of the circuits 150 represented by two-dot chain lines, a plurality of printed wiring boards 200 are obtained from the single laminated board 190.
Note that, to minimize length of lines thereof, the circuit 150 is generally constituted by straight lines as a basic pattern. Besides, in order to increase a density of arrangement of the circuits 150, a straight-line part of the circuit 150 is often formed so as to be parallel to an outer side of the printed wiring board 200 with a rectangular shape. As a result, a straight-line part 151 of the circuit 150 is probably parallel to the warp threads 110 of the cloth 130, and a straight-line part 152 of the circuit 150 is probably parallel to the weft threads 120 of the cloth 130 (in each of FIGS. 6B and 6C, an arrow “X” shows a direction of the warp threads 110 and an arrow “Y” shows a direction of the weft threads 120). In this regard, when the warp threads 110 and the weft threads 120 of the cloth 130 embedded in the printed wiring board 200 as the substrate are provided parallel to the straight-line parts 151 and 152 of the circuit 150, respectively, the following problem is likely to occur.
FIG. 7 is an enlarged cross-sectional view illustrating the printed wiring board 200 obtained by forming the circuit 150 on the face of the laminated board 190. The laminated board 190 is formed in such a manner that the cloth 130 is embedded in the resin 140. Besides, FIG. 7 is the cross-sectional view cut along a vertical plane to the longitudinal direction of the warp thread 110 and shows circuits 150 which are provided parallel to the warp threads 110. Out of the circuits 150 provided parallel to the warp threads 110 as described above, some of the straight-line parts of the circuits 150 are disposed just above the warp threads 110 and the others are disposed above a space between the warp threads 110 and 110.
Glass cloth is typically used as the cloth 130, and a glass fiber of an inorganic substance has a high dielectric constant while resin has a low dielectric constant. In the laminated board 190 prepared by impregnating the cloth 130 with the resin 140 and subsequently hardening the resin 140, a portion including the warp threads 110 of glass fibers and the weft threads 120 of glass fibers has a high dielectric constant. In contrast, a portion between the warp threads 110 and 110 and/or between the weft threads 120 and 120 dominantly includes the resin 140 and thus has a low dielectric constant. Namely, the dielectric constant is greatly changed depending on the portions of the laminated board 190. As described above, the entire straight-line part of the circuit 150 disposed just above the warp thread 110 extends parallel and close to the warp thread 110. Therefore, a transmission rate by use of this circuit 150 as a signal line is affected greatly by the warp thread 110 with the high dielectric constant. In contrast, the entire straight-line part of the circuit 150 disposed above the space between the warp threads 110 is far from the warp threads 110, and therefore is more likely to be affected by the close resin 140. Hence, when this circuit 150 is used as a signal line, the transmission rate is affected greatly by the resin 140 with the low dielectric constant.
As described above, when the circuits 150 are provided parallel to the warp threads 110, a problem occurs that a difference occurs between the signal transmission rates of the circuit 150 extending along and just above the warp threads 110 and the circuit 150 extending along and above the space between the warp threads 110 and 110. A similar problem occurs in the circuits 150 provided parallel to the weft threads 120. When the circuits 150 are provided parallel to the weft threads 120, this would cause a problem that a difference between signal transmission rates of the circuit 150 extending just above and along the weft threads 120 and the circuit 150 extending along and above the space between the weft threads 120 and 120.
In particular, differential transmission can reduce signal amplitude compared with single ended transmission, and thus the differential transmission can increase a data transmission rate. The differential transmission is a method of transmitting data using paired signal lines. Accordingly, it is important to reduce a difference between the transmission rates.
In this regard, in Patent Document 1, when used is cloth composed of warp and weft threads made of glass fiber bundles, the fiber bundles of the warp and weft threads are subjected to opening treatment so as to be flat, in order to reduce gaps between the warp threads and between the weft threads. In this way, when the gaps between the warp threads and between the weft threads are reduced, the amount of the resin present in the gaps decreases, and therefore uneven distribution of the warp threads, the weft threads, and the resin decreases. Hence, if the circuits are provided parallel to the warp threads or the weft threads and some of the circuits extending just above and along the warp threads or the weft threads and the others are extending above and along the space between the warp threads or between the weft threads, it is possible to reduce the unevenness of the dielectric constants that affects the signal transmission rates of these circuits.
However, in this case, uneven distribution of the warp threads, the weft threads, and the resin cannot be small enough, and an effect for reducing the unevenness of the dielectric constant that affects the signal transmission rates of these circuits, is not provided so as to be expected. Besides, in Patent Document 1, high-pressure water jet or the like is required for the opening treatment, and therefore there is a problem that production equipment is required.
In this regard, in order to solve the above problem caused by that the circuits 150 are provided parallel to the warp threads 110 or the weft threads 120 of the cloth 130, there has been proposed to prepare the printed wiring board 200 by positioning the printed wiring board 200 oblique to the laminated board 190 as shown in FIG. 6C. Namely, the printed wiring board 200 is prepared to be oblique to the laminated board 190 by forming the circuits 150 on the face of the laminated board 190 in such a manner that the straight-line parts 151 of the circuits 150 are not parallel but oblique at about 10 degrees to the side edge “e” along the longitudinal direction of the laminated board 190 and the straight-line parts 152 of the circuits 150 are not orthogonal but oblique at about 10 degrees to the side edge “e” along the longitudinal direction of the laminated board 190.
In this way, when the straight-line parts 151 and 152 of the circuits 150 are formed so as to be oblique to the side edge “e”, the straight-line part 151 of the circuit 150 is not parallel to the warp threads 110 of the cloth 130, and the straight-line part 151 of the circuit 150 extends across the warp threads 110 and the space between the adjacent warp threads 110 and 110 in turn. Similarly, the straight-line part 152 of the circuit 150 is not parallel to the weft threads 120 of the cloth 130, and the straight-line part 152 of the circuit 150 extends across the weft threads 120 and the space between the adjacent weft threads 120 and 120 in turn. Accordingly, the straight-line parts 151 and 152 of the circuit 150 are to be affected by both of the resin 140 with the low dielectric constant and the warp threads 110 or the weft threads 120 with the high dielectric constant in a similar way. Since the case is less likely to occur where some of the circuits 150 are affected by the warp threads 110 or the weft threads 120 with the high dielectric constant and the others of the circuits 150 are affected by the resin 140 with the low dielectric constant, the difference between the dielectric constants is less likely to affect the signal transmission rates, and it is possible to prevent occurrence of the difference between the signal transmission rates with using the circuits 150.
However, when the circuits 150 are formed so as to be oblique to the laminated board 190 on the face thereof as shown in FIG. 6C, degree of freedom in patterning wire for the circuit 150 is reduced, and therefore a problem may occur that it is difficult to increase a wiring density. Besides, since the printed wiring board 200 is prepared so as to be oblique to the laminated board 190, with regard to a process of forming a plurality of printed wiring boards 200 from a single laminated board 190, a problem may occur that an efficiency of forming the plurality of printed wiring boards 200 from the single laminated board 190 decreases, for example, and the blank area increases.